Problem with transmission of a material that uses principled shader
Go to the Material Tab. Scroll down to &quot;Settings&quot; and Check &quot;Screen Space Refraction&quot;

You also need to enable &quot;Refraction&quot; in the &quot;Screen Space Reflections&quot; panel of the render properties. Render engine must be set to Eevee beforehand.

Also make sure the roughness value is not too high, above 0.5 it will gradually prevent refraction.